Thermal management of various electronic and opto-electronic devices is increasingly gaining attention due to the severe challenges faced in such devices. The trend of shrinking sizes and increased functionality continues in personal hand-held electronic devices. The power density, and hence the density of heat that needs to be dissipated have significantly increased, which poses significant challenges to providing good thermal management in those devices. Similarly, in opto-electronic devices, also known as light emitting diodes (LEDs), the power consumption and lumen output is ever increasing. Thermal management problems are also widely prevalent in other applications such as electronic components in automobiles, rechargeable battery systems and power inverters for hybrid vehicles, etc. Insufficient or ineffective thermal management can have a strong and deleterious effect on the performance and long-term reliability of devices.
Currently LED-based bulbs are being used to replace older bulbs and are designed to fit into conventional “Edison” sockets. Fitting LED bulbs into Edison sockets only exacerbates the thermal management challenges since the heat dissipation is limited by natural convection. LED bulbs therefore require well-designed heat sinks to efficiently and adequately dissipate the waste heat. Inefficient thermal management leads to higher operating temperatures of the LEDs, which is indicated by the junction temperature (Tj) of the LED. The lifetime of an LED (defined as time taken to lose 30% light output i.e. reach B70) can potentially decrease from 80,000 hours to 20,000 hours when the junction temperature is increased from 115° C. to 135° C.
Aluminum heat sinks are a natural choice for LED applications based on similarities to heat sinks used for other electronic devices. However the use of aluminum heat sinks for LED bulbs presents several challenges. One challenge is electrically insulating the heat sink from the Edison socket. Any electrical connectivity or leak between a metal heat sink and the socket can be extremely dangerous during installation. Another challenge is providing heat sinks with complex shapes because die-casting heat fin shapes can be difficult and may require costly secondary machining operations. Aluminum heat sinks can also be quite heavy and can add significantly to the weight, and hence cost of transportation, of the bulb. Finally, aluminum heat sinks will need a finish step of painting to smooth surface finish and impart colors desired by the consumers.
Plastics can be an attractive alternative to aluminum for heat sinks. Plastics are electrically insulating, more amenable to complex heat sink structures via injection molding, light in weight, and can be colored freely to meet aesthetic or branding requirements. Plastics also offer the possibility of integrating several parts, which can lead to a simpler overall assembly of the bulb. Plastics, however, have very poor thermal conductivity—generally only around 0.2 W/mK—which is nearly two orders of magnitude lower than that of typical die-cast aluminum alloys (which are around 200 W/mK). Therefore, plastics are generally not sufficient to meet thermal management challenges.
Fillers are often added to plastics to make unique composite materials. For example, reinforcing fillers like glass fibers are added to improve the mechanical properties of plastics. Similarly graphite, carbon black or other carbon forms, including even carbon nanotubes recently are added to plastics to make electrically conductive plastic-based materials. Graphite and metal powders are also used sometimes to enhance thermal conductivity, but this usually leads to increased electrical conductivity as well since these properties are usually concomitant. However, some ceramic materials such as silica, alumina, zinc oxide, magnesium oxide, aluminum nitride, boron nitride (hexagonal or cubic forms), etc. present the opportunity to make thermally conductive yet electrically insulating formulations with plastics since they are good thermal conductors and electrical insulators.
While boron nitride plastic composites have been proposed, boron nitride/plastic composites have several drawbacks. Boron nitride is a relatively expensive material that can cost from 5 to 40 times more than the plastic resins that it is compounded with and as compared to aluminum alloys. From a performance standpoint, the in-plane thermal conductivity of the boron nitride/plastic composite is only around 2-10 W/mK even at high loadings of boron nitride, e.g., above 25-60 wt. % (15-45 vol %). Boron nitride is also very inert and not easily wet by resins. This leads to imperfect interfaces and large thermal resistances between the filler and matrix, effectively lowering the thermal conductivity of the composite thus leading to higher BN loadings required to achieve the required thermal conductivity. The higher filler loadings drives up the cost of these composites significantly making it less cost competitive in thermal management applications. The poor interfaces between the filler and resin also results in poor physical properties of the composites. It therefore becomes imperative to address the problems of wetting to achieve high thermal conductivity and optimum physical properties.
It is important to note however that even though thermal conductivity of thermally conductive plastics is not as high as aluminum metal, it is sufficient for thermal management applications in LED bulbs, and other convection limited applications. The inherent anisotropy of boron nitride/plastics composites can also be an issue which may limit the applicability of boron nitride/plastic composites in some applications where the through-plane thermal conductivity is critical to the application.